To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the well and the guns fired to create openings in casings and to extend perforations into the surrounding formation.
The explosive nature of the formation of perforation tunnels shatters sand grains of the formation. A layer of “shock damaged region” having a permeability lower than that of the virgin formation matrix may be formed around each perforation tunnel. The process may also generate a tunnel full of rock debris mixed in with the perforator charge debris. The extent of the damage, and the amount of loose debris in the tunnels may impair the productivity of production wells or the injectivity of injector wells.
To obtain clean perforations and to remove perforation damage, underbalanced perforating can be performed, where the perforation is carried out with lower wellbore pressure than the formation pressure. Schlumberger's PURE (Perforating for Ultimate Reservoir Exploitation) technology has been used to provide a transient underbalance just after creating perforations to minimize or eliminate perforation damage and to enhance productivity or infectivity.
However, it has been determined that using just a transient underbalance does not provide optimal perforations in some scenarios.